The goal of the proposed studies is to use functional magnetic resonance imaging (fMRI) to identify brain regions corresponding to behaviorally observed inhibitory deficits in older adults. The critical question is whether there is one superordinate region or system which coordinates inhibitory behavior, such as the prefrontal lobes (West, 1996) or frontal-subcortical circuits (Alexander et al., 1986). Alternatively inhibition may be coordinated within each response modality and there is no overlapping superordinate system. It will be assumed that behavioral inhibition involves suppression of a prepotent/reflexive in favor of an intentional response. The first series of experiments will compare the overlap in functional activation for varied inhibitory tasks in young and elderly adults. The second experiments will test young and elderly adults under conditions which impair and facilitate inhibitory behavior, specifically at varying points in circadian the arousal cycle. The third experiment relates inhibitory behaviors to other cognitive processes by comparing subgroups of subjects that have been selected based on performance on a test of working memory functioning.